As previously described, we have established a model system to study the damage to foreign cell membranes which occurs when killer lymphocytes attack them. Antibody coated lipid bilayers serve as the target membranes for attack by normal human lymphocytes. Electrical measurements show that the conductance, and hence ionic permeability increases by up to several orders of magnitude when lymphocytes are added under conditions which allow killing. In order to define the nature of the lesions inflicted by the attacking lymphocyte, the ionic selectivity of membranes whose conductance was increased 20-100x was studied. This was accomplished by increasing the concentration of various salts on one side of the membrane and observing the membrane potential. No potentials were induced by a series of salts whose cations ranged as large as tetraethylammonium and whose anions ranged as large as cacodylate. Thus the lymphocytes alter the lipid bilayer conductance in a manner very different from previously studied ionophores, and it was concluded that the lesions induced by killer lymphocytes are large enough to allow all such ions to pass freely through the membrane. Non electrical methods must be developed in order to study the size of the presumed channels.